Process of bleaching wood pulp by combined treatment with peroxide and an alkali metal borohydride



states The present invention relates to the bleaching of wood pulp using a novel combination of reducing and oxidizing agents. More particularly, the present invention relates to the bleaching of wood pulp wherein the pulp is subjected to the combined and simultaneous action of peroxide and borohydride.

The present invention relates to the bleaching of Wood pulps in general, but has particular application to mechanically disintegrated Wood pulps. The term mechanically disintegrated wood pulp embraces both ground wood pulp and chip-mechanical pulp; the former is obtained by well-known methods of reducing logs to fibers by grinding and the latter is obtained by refining hardwood or softwood chips with chemicals, such as sodium bisulfite or caustic soda With or without pressure, elevated temperatures or merely treating the chips with steam, hot or cold water, and thereafter grinding the chips to fibers. The bleaching of mechanically disintegrated wood pulps began over a decade ago using a single peroxide treating step. Thus bleached, the pulp can be used in varymg amounts in paper finish to improve the printability and other characteristics of the paper. Increasing demands for higher brightness of mechanically disintegrated wood pulp has resulted in increased efiiciency in bleaching processes. More recently instead of bleaching this pulp in a single stage, commercial practice has adopted the procedure of initially bleaching the pulp with peroxide, neutralizing the slurry and thereafter subjecting the slurry to the action of a reducing agent, usually hydrosulfite. Such a two-stage procedure is well known having been used for several years.

The same trend to up-grade pulp has taken place in the chemical pulp field. Instead of a single hypochlorite bleaching stage, common practice today is to bleach such pulp in five or six stages. A conventional bleaching procedure is to employ a five-stage process where the pulp is subjected to chlorination, caustic treatment, the action of hypochlorite, chlorine dioxide treatment, and thereafter a final peroxide bleach. US. Patent 2,494,542 relates to such a process. This and other multi-stage bleaching procedures result in producing chemical pulp of high brightness and superior brightness stability.

In the instance of mechanically disintegrated Wood pulp, the recently developed two-stage procedure initially provides for the pulp to be subjected to an alkaline bleaching solution containing peroxide. Thereafter the slurry is neutralized with sulfur dioxide While the residual peroxide content is sufficiently high (at least about 5%, preferably about -:25%) to prevent caustic color reversion. The usual practice of treating the pulp in the peroxide bleaching cycle is to introduce peroxide under alkaline conditions and bleach the pulp for the required time. After neutralization the pulp slurry is treated with a reducing agent, usually a metal hydrosulfite and a chelating agent.

It has been found that when borohydride is added to the pulp during the initial peroxide bleaching stage in the conventional two-stage process, a marked increase in brightness gain is realized. Thus treated, it is unnecessary to treat the pulp with hydrosulfite. Subsequent treatment with hydrosulfite is optional, however. It has also been found that if the borohydride compound is added 3,lh0,732 Patented Aug. i3, 1963 at the end of the conventional peroxide bleaching stage, the residual peroxide will be utilized and an even greater increase in brightness gain can be realized. The use of borohydride at the end of the peroxide bleaching stage confers the added advantage of effectively eliminating residual peroxide; it is then no longer necessary to use hydrosulfite in .a subsequent stage. Additionally it is possible to neutralize the slurry with sulfuric acid rather than neutralizing with the more expensive sulfur dioxide. Although both borohydride and peroxide have been used to bleach wood pulp through reduction and. oxidation actions respectively, so far as known these agents have not been used in combination.

It is an object of the present invention to provide an improved process of bleaching both mechanically disintegrated and chemical pulp by the combined and simultaneous actions of an oxidizing agent and a reducing agent.

It is another object of the present invention to provide a process for effectively utilizing the residual peroxide remaining in pulp slurries before neutralization.

It is still another object of the present invention to provide a process wherein alkaline pulp slurries can be neutralized by sulfuric acid.

These and other objects will be apparent from the following description.

The instant invention relates to (l) the use of peroxide and borohydride in admixture to bleach mechanically disintegrated or chemical pulp and (2) to the use of borohydride as a post treatment to effectively utilize the residual peroxide remaining at the end of the peroxide bleaching cycle. In both applications, superior results are obtained by the combined and simultaneous action of an oxidizing agent and a reducing agent.

. In the first of the above procedures, an alkali metal borohydride, preferably sodium borohydride, is dissolved in the peroxide solution containing suitable bulfering and stabilizing agents, such as Epsom salt, sodium silicate, sodium tripolyphosphate (or ethylene diamine tetracetic acid as a chelating agent), sodium carbonate and caustic soda, immediately before the solutionis mixed with the pulp. The pulp slurry, which can have a consistency of about 4-30%, is retained for completion of the reaction at a temperature of about -480 F. for a period of 15 minutes up to six hours or longer if necessary. The

length of the treating period largely depends on consistency and temperature. At the end of the bleaching cycle, the pulp is neutralized with sulfuric acid to about pH 6.5-7 or lower. The presence of borohydride insures that little or no residual peroxide remains.

In the second of these applications the peroxide bleaching stage is carried out in the conventional manher at the above-indicated consistency and temperature. At the end of the peroxide bleaching stage, the pulp is preferably reduced in consistency to about 4 or 5% by adding water. Borohydride is then added to the pulp and the slurry is retained at about 90-l30 F. for onehalf to two hours 101' longer and thereafter neutralized with sulfuric acid. Alternatively, the acid can be added at about the same time as the borohydride is introduced into the slurry. P ulp so treated can then be bleached at neutral or slightly acid pH (about 6) and need not be further neutralized; after bleaching the pulp is ready for paper furnish use. It is actually unnecessary to reduce the consistency of the pulp before adding boro hydride, adequate mixing permits the use of pulps having consistencies of 30% or more. The amount of boro hydride can be as high as economical and brightness requirements demand. Broadly, (Ll-3%, calculated as sodium borohydride, is satisfactory and preferably 0.5- 1% is used.

Throughout the specification and examples percentages are by weight based on the dry weight of the pulp. Although sodium borohydride is preferred, potassium and other alkali metal borohydrides are suitable in the [form of water stable solutions, powder or pellets, preferably in powder lform. The preferred peroxide is an alkali metal peroxide, specifically sodium peroxide, but 0.12.0% of any conventional peroxide based on sodium peroxide can be used. Thus hydrogen peroxide is suitable in amounts up to Preferably 0.5-2% peroxide, based on sodium peroxide, is used.

A batch of ground wood pulp was dewatered to 14% consistency and bleached with a conventional 2% sodium peroxide formula with a resultant brightness increase of 8.5 G.E. units after neutralization. Two additional samples were bleached in the same manner except that in one case 0.1% sodium borohydride was dissolved in the peroxide solution and, in the second case, 1.0% sodium borohydride was added. The brightness increase using 0.1% sodium borohydride with the peroxide solution was 9.7 GB. units, or 1.2 G.E. units greater than peroxide alone. When the 1.0% sodium borohydride was used, the brightness increase was 12.4 G.E. units, or approximately 4 G.E. units greater than peroxide alone. All bleaching trials were carried out at 110 F. for 90 minutes.

The following experiment illustrates the effect that borohydride has when added to unbleached chemical pulps being subjected to an alkaline peroxide bleach.

Samples of sulfate (kraft) pulp were prepared; the first solution contained 0.21% hydrogen peroxide (equivalent to 0.5% sodium peroxide), 5% sodium silicate, 0.8% caustic soda and 0.05% Epsom salt. The second and third solutions were the same except that in one solution 0.1% sodium borohydride was added and, in the other solution, 0.5% sodium borohydride was added.

Bleaching of the pulp samples was carried out at 12% consistency for 40 minutes at 130 F. The pulp bleached with peroxide alone had a brightness increase of 4.8 6.13. units. The solution containing 0.1% sodium borohydride bleached pulp to an increase brightness gain of 7.0 G.E. A gain of 10.7 G.E. units was obtained with the combined use of peroxide and 0.5% sodium borohydride.

Following the process described in U.S. Patent 2,494,542, chemical pulp is treated with conventional bleaching agents and the pulp is next subjected to the action of chlorine dioxide and then an alkaline peroxide bleach. During the last stage while the pulp is being treated with peroxide under alkaline conditions, borohydride is added at about the same time as the peroxide. Alternatively, the borohydride can be added subsequent to the addition of peroxide providing residual peroxide is about 5% or higher. It is only necessary that the peroxide and borohydride be present at the same time.

Using borohydride as a post treatment of a pulp slurry in the peroxide stage, a sample of ground wood was bleached with a conventional 2% sodium peroxide (formula at a consistency of 12% for 80 minutes at 110 F. The brightness gain after neutralization with sulfiur neutralized with S0 dioxide and sulfuric acid in the conventional manner gave a 9.4 GB. unit brightness increase. A portion of the pulp in the peroxide stage containing 27% residual peroxide was diluted to 5% consistency and 1.0% sodium borohydride was added. 'Fhe pulp was retained at F. [for minutes and then neutralized to a pH of 4.5 with sulfuric acid. The brightness gain was 14.4 G.E. units, or 5 units more than the pulp bleached by peroxide alone. The pH :of the pulp prior to neutralization with sulfuric acid was 9.8 and there was no residual peroxide in the pulp.

The relative brightness stability of the pulp bleached with peroxide alone, peroxide and borohydride in admixture, orwith borohydride as a post treatment was the same in all experiments.

The combination of peroxide and borohydride in either the initial alkali peroxide bleaching cycle or as a post treatment to utilize residual peroxide can be applied in presently used peroxide bleacheries using conventional equipment. The superior results obtained were unexpected since a mixture of an oxidizing agent and a reducing agent would be expected to have an adverse efiect on bleaching.

The present invention finds particular application in connection with mechanically disintegrated pulp. An added advantage of using borohydride in the alkaline peroxide bleaching cycle is that the pulp need not be Because there is little or no residual peroxide in the pulp, sulfuric acid is suitable as a neutralizer.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A method of bleaching wood pulp comprising subjecting the pulp to the combined and simultaneous action of peroxide and an alkali metal borohydride.

2. A method of bleaching mechanically disintegrated wood pulp wherein said pulp has been subjected to an alkaline peroxide bleach and contains suflicient residual.

peroxide to prevent caustic color reversion, the step com prising adding 0.13% by weight of an alkali metal borohydride to said pulp and thereafter neutralizing said pulp.

3. The method of claim 2 wherein the amount of borohydride is 0.Sl%.

4. The method of claim 2 wherein said neutralization step is accomplished with sulfuric acid.

5. A method of bleaching mechanically disintegrated Wood pulp comprising subjecting said pulp to the combined and simultaneous action of an alkaline peroxide bleaching solution, said solution containing peroxide equivalent to 0.1-2% sodium peroxide and an amount of borohydride equivalent to 0.13% sodium borohydride, and thereafter neutralizing the pulp with sulfuric acid.

6. The method of claim 5 wherein the amount of peroxide is equivalent to 0.52% sodium peroxide and the amount of borohydride is equivalent to 0.5-1% sodium borohydride.

References Cited in the file of this patent TAPPI, vol. 38, No. 11, Nov. 19, 1955, pages 682687. 

1. A METHOD OF BLEACHING WOOD PULP COMPRISING SUBJECTING THE PULP TO THE COMBINED AND SIMULTANEOUS ACTION OF PEROXIDE AND AN ALKALI METAL BOROHYDRIDE. 